The goal of this project is to compare the effectiveness of limited resection vs. stereotactic body radiotherapy (SBRT) for the treatment of stage IA non-small cell lung cancer (NSCLC). NSCLC is the leading cause of cancer deaths in the US. However, approximately 25% of patients are diagnosed with early, potentially curable disease. While the standard treatment for stage I NSCLC is lobectomy, full resection is precluded in many patients due to limited lung function or high burden of comorbidities. These patients are managed with less aggressive, but still quite effective, limited lung resections. More recently, SBRT, has emerged as an alternative treatment for high operative risk patients with tumors ?5 cm in size. Despite rapid adoption, evidence supporting the use of SBRT is very limited. Phase I-II trials showed that SBRT is associated with 3- year survival rates comparable to limited resection. However, these studies likely overestimated the effectiveness of SBRT as they included patients without biopsy proven cancer. Moreover, all phase III studies evaluating SBRT have been closed prematurely due to poor accrual. Limited information is also available about adverse events (AEs) and quality of life (QOL) following these treatments. Unfortunately, this knowledge gap is a major challenge in selecting the best treatment for patients at high risk for lobectomy and to translates into poorer outcomes. The Specific Aims of the study are to: 1) compare overall (primary outcome), lung cancer-specific and disease-free (secondary outcomes) survival of patients with stage I NSCLC ?5 cm in size treated with limited resection vs. SBRT; 2) evaluate rates of severe AEs among patients with stage I NSCLC ?5 cm in size treated with limited resection vs. SBRT; and 3) assess differences in lung function and QOL among patients with stage I NSCLC treated with limited resection vs. SBRT. To achieve these Aims, we will conduct a multisite prospective observational study of 450 patients with clinical stage I NSCLC ?5 cm in size who are high risk for lobectomy and thus, being consider for limited resection or SBRT. Management of these patients (including treatment decisions) will be conducted according to routine care. Participants will undergo a comprehensive baseline (pre-treatment) evaluation to collect data about patient and lung cancer characteristics and will be followed at 7- and 30-days, 3- and 6-months, and 1-year of treatment to collect data on the surgical or SBRT regimen, AEs, lung function, and QOL. We will interview the treating physicians and review medical records to obtain information about the treatment plan, complications, and cancer recurrence. Survival will be assessed at the end of the study via a National Death Index search. We will use propensity scores and instrumental variable analysis to compare outcomes of patients treated with limited resection vs. SBRT while controlling for allocation bias. The study will rapidly provide practical information to guide treatment decisions and will inform policy recommendations regarding the use of these potentially life-saving therapies.